Ghrelin was identified as the natural ligand of the growth hormone secretagogue receptor 1a (GHSR1a). The receptor is most abundant in the pituitary gland and in hypothalamic parts of the brain, but can also be detected in other tissues at low concentrations. Since the late 70ies synthetic peptides and other compounds, named secretagogues had been shown to stimulate the release of growth hormone. However, the natural ligand responsible for the release of growth hormone remained unknown until the discovery of ghrelin in 1999. Ghrelin is a highly basic 28 amino acid peptide hormone with an octanoyl acid side chain at the third amino acid of its N-terminus (serine 3). This unusual modification is required for the interaction at the GHS-receptor and its activity. However, in biological samples a mixture of both the octanoyl ghrelin which is a form of a bioactive ghrelin, and the unmodified or des-octanoyl ghrelin is present. The amino-acid sequence of the purified rat ghrelin was determined to be GSSFLSPEHQKAQQRKESKKPPAKLQPR (SEQ. ID. No. 2); the corresponding human sequence deviates in two positions only, carrying the same n-octanoyl-side chain at the amino acid position serine 3 and was determined to be GSSFLSPEHQRVQQRKESKKPPAKLQPR (SEQ. ID. No. 1).
Beside the naturally occurring n-octanoyl residue, unsaturated or branched octanoyl groups, and longer aliphatic chains introduced at position 3 of ghrelin mediate receptor recognition as well. The receptor interaction domain is located at the very N-terminus of ghrelin; deletion studies indicate, that the minimal motif of amino acids 1-5 (ghrelin (1-5) [GSSFL]) (SEQ ID NO: 1, amino acids 1-5) is sufficient for stimulation of GHSR1a, but a strong requirement for peptide modification with the n-octanoyl residue is observed.
Ghrelin has been shown to mediate physiological functions pertinent to an anabolic state. It directly stimulates the release of growth hormone (GH) from the pituitary gland, and may therefore be a suitable target in the treatment of acromegaly. Experiments in rodents also showed ghrelin to induce feeding in a GH-independent fashion by acting upon hypothalamic neurons. Interestingly, the primary site of ghrelin production is in oxyntic glands in the stomach, suggesting that it serves as a hormonal link between stomach, pituitary gland and hypothalamus. The observation that ghrelin administration in rats resulted in weight gain as a consequence of changes in energy intake and/or fuel utilization is in support of such a role. Moreover, systemic ghrelin administration in humans cause sensations of hunger in the test subjects and induce overeating. Based on these findings ghrelin is thought to have a crucial role in the regulation of appetite and body weight, serving as an acute as well as a chronic signal of an underfed state. Additional support for this hypothesis comes from observations that ghrelin levels as well as appetite are reduced in individuals following gastric bypass, contributing at least in part to the efficiency of the procedure in effecting weight loss. Clinical data from patients with Prader-Willi syndrome also suggest that the hyperphagia and obesity associated with the disease are a consequence of tremendous hyperghrelinemia. Moreover, ghrelin was found to induce hyperglycemia and inhibition of insulin release, indicating an involvement in glucose metabolism. Beside these functions in energy metabolism, ghrelin has also been implicated in a number of other processes in the field of gastrointestinal diseases, such as gastric emptying and regualtion of bowel movements. Moreover, ghrelin was also found to be expressed in a number of neuroendocrine tumors and to stimulate, besides GH release from the pituitary, the release of ACTH, PRL, and cortisol. Single injections of ghrelin into healthy individuals were found to increase cardiac output and decrease blood pressure. Thus, ghrelin action appears to be involved in a variety of different tasks. Additional background information related thereto can be found in M. Kojima, H. Hosoda, Y. Date, M. Nakazato, H. Matsu, K. Kangawa, “Ghrelin is a growth-hormone-releasing acylated peptide from stomach”, Nature 402:656-60, 1999; M. Tschöp, D. L. Smiley, M. L. Heiman, “Ghrelin induces adiposity in rodents”, Nature 407:908-13, 2000; A. M. Wren et al., “Ghrelin enhances appetite and increases food intake in humans”, Journal of Clinical Endocrinology Metabolism 86:5992-6, 2001; M. Nakazato et al., “A role for ghrelin in the central regulation of feeding”, Nature 409: 194-8, 2001; N. Nagaya, et al., Am J Physiol Regul Integr Comp Physiol. May 2001; 280(5):R1483-7; Hemodynamic and hormonal effects of human ghrelin in healthy volunteers; Volante M, et al., J Clin Endocrinol Metab. March 2002; 87(3):1300-8. Expression of ghrelin and of the GH secretagogue receptor by pancreatic islet cells and related endocrine tumors; Jeffery P L, et al., J Endocrinol. March 2002; 172(3):R7-11 Expression and action of the growth hormone releasing peptide ghrelin and its receptor in prostate cancer cell lines; Egido E M, et al., Eur J Endocrinol. February 2002; 146(2):241-4 Inhibitory effect of ghrelin on insulin and pancreatic somatostatin secretion; Broglio F, et al., J Clin Endocrinol Metab. October 2001; 86(10):5083-6, Ghrelin, a natural GH secretagogue produced by the stomach, induces hyperglycemia and reduces insulin secretion in humans; Bednarek M A, et al., J Med Chem. October 2000; 43:4370-6 Structure-function studies on the new growth hormone-releasing peptide, ghrelin: minimal sequence of ghrelin necessary for activation of growth hormone secretagogue receptor 1a.
The problem underlying the present invention is to provide a specific antagonist to ghrelin. A further aspect of the problem underlying the present invention is to provide a specific antagonist to the growth hormone secretagogue receptor 1 a (GHSR 1a). Another aspect of the problem underlying the present invention is to provide a compound for the treatment of diseases and disorders involving ghrelin and the GHSR 1a receptor, respectively.
A further problem underlying the present invention is to provide means for the binding of bioactive ghrelin and more particularly to provide a method for the treatment of diseases and disorders mediated by bioactive ghrelin as well as methods for the specific detection of bioactive ghrelin.